1. Field of the Invention
The present invention relates to a saw and specifically, to a hole saw, which is practical to cut countersinking holes of different shapes and sizes.
2. Description of the Related Art
FIGS. 1 and 2 show a hole saw according to the prior art adapted to cut a stepped circular hole out of a wooden or metal plate member. According to this design, the hole saw comprises a hole-cutting saw blade device 50, a pilot drill 60, and a shaft 70. The hole-cutting saw blade device 50 comprises an outer cutter 52 and an inner cutter 54. The outer cutter 52 has an inner periphery abutted with the outer periphery of the inner cutter 54. Further, the outer cutter 52 has a plurality of cutting teeth 521 equiangularly spaced around the bottom of the outer cutter 52. The inner cutter 54 has a series of cutting teeth 541 extending around the bottom thereof. The pilot drill 60 has the head thereof fastened to the center hole of the shaft 70. When in use, the pilot drill 60 is rotated with the shaft 70 and driven into the workpiece to guide the cutting working of the hole-cutting saw blade device 50, and then the cutting teeth 541 of the inner cutter 54 are acted against the workpiece to cut a circular hole A out of the workpiece, and finally the cutting teeth 521 of the outer cutter 52 are acted against the workpiece to cut a circular hole B greater than the circular hole A, thereby forming a stepped circular hole.
According to the aforesaid prior art design, the hole-cutting saw blade device 50 comprises an outer cutter 52 and an inner cutter 54 fixedly arranged together. When the cutting teeth 541 of the inner cutter 54 or the cutting teeth 521 of the outer cutter 52 have sworn or are damaged during working, the outer cutter 52 and the inner cutter 54 cannot be separately replaced, and the whole hole-cutting saw blade device 50 must be replaced. Further, because the inner diameter of the outer cutter 52 must fit the outer diameter of the inner cutter 54 perfectly, different sizes of outer cutters 52 must be matched with different sizes of inner cutters 54 to set up a full range of hole-cutting saw blade devices. These problems greatly increase the cost.